I. Introduction
This invention relates to an improved corrosion resistant phosphate coating for parts fabricated from iron and steel.
2. Description of the Prior Art
As is known in the art, phosphate coatings are conversion coatings for iron and steel. The coatings serve as a base for organic coatings to improve wear resistance and/or impart color to the base metal and to provide corrosion resistance to the base metal. For the most part, the coatings are mixed phosphates of the metals comprising the phosphating solution (the primary metal) and of iron from the base metal. Formation of a phosphate coating is by contact of the base metal with a phosphating composition for a time and at a temperature necessary to provide a coating of the desired thickness. Methods and compositions for phosphating are well known and disclosed in numerous publications including, for example, the Forty-Fourth Annual Edition of the Metal Finishing Guidebook and Directory, Metal and Plastics Publications, Inc., Hackensack, N.Y. 1976, pages 554 to 566; Burns and Bradley, Protective Coatings for Metals, Reinhold 1967, Third Edition, pages 568 through 575; and U.S. Pat. Nos. 2,164,042; 2,326,309; 2,351,605; 3,118,792 and 4,168,983, all incorporated by reference.
Compositions for phosphating a surface typically comprise a dilute aqueous acidic solution of a metal phosphate formed by the dissolution of a primary metal salt in phosphoric acid, phosphoric acid and an oxidizing agent as an accelerator. The metal salt dissolved in the phosphoric acid is most often zinc oxide with the formation of a primary zinc phosphate coating, but salts of manganese and iron are often used either alone or in combination with the zinc oxide. The phosphate coating is formed by free phosphoric acid attacking the metal surface liberating iron which goes into solution thus providing iron phosphate in solution in addition to the primary metal phosphates. At the interface of a base metal surface and the solution, the pH is altered resulting in the insolubilization of the phosphates and precipitation of the same on the surface of the base metal forming the conversion coating. An overall reaction for formation of the coating can be written as follows using zinc as illustrative of the primary metal in the solution:
3Zn(H.sub.2 PO.sub.4).sub.2 +Fe+4H.sub.2 O.fwdarw.Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O+FeHPO.sub.4 +3H.sub.3 PO.sub.4 +3H.sub.3 PO.sub.4 +H.sub.2
The combination of zinc and iron phosphates in the above equation represent the phosphate coating.
Though phosphate coatings have been used for many years to improve corrosion resistance of a part formed from iron or steel, further improvements are desired. One such improvement known to the art involves use of a secondary treatment solution. For example, it is known to treat a phosphate coated surface with an aqueous solution of a stannous salt which is water soluble and water stable. A preferred solution comprises an aqueous solution of stannous chloride as disclosed in U.S. Pat. No. 2,478,954. An improvement in the method disclosed in U.S. Pat. No. 2,478,954 is disclosed in U.S. Pat. No. 3,118,792 where corrosion resistance is further improved by immersion of a sheet of lead in the stannous salt solution which is believed to be a source of lead chloride formed by neutralization of hydrochloric acid formed during reaction. Other improvements to corrosion resistance imparted by phosphating solution are accomplished by additives in the primary phosphate solution. One such additive is a cyclic trimeta phosphate as disclosed in U.S. Pat. No. 4,168,983.
Notwithstanding the improvements in the art of phosphating described above, corrosion of iron and steel is a major problem and further improvements are desired.